Manufacture of fuel for nuclear reactors utilizing a polymerisable resin

ABSTRACT

1. A METHOD OF MAKING PRESSED COMPACTS OF NUCLEAR FUEL RESIDING IN PREPARING A MIXTURE INCLUDNG GISSION PRODUCT RETANING FUEL PARTICLES, A SOLID POLYMERISABLE RESIN AND A GRANULAR MATRIX MATERIAL, LOADING A DIE CAVITY WITH THE MIXTURE, HEATING THE DIE AND CONTENTS OF THE CAVITY SO THAT A UNIFORM TEMPERATURE EXISTS THROUGHOUT THE CONTENTS, BEING A TEMPERATURE AT WHICH THE RESIN MELTS BUT DOES NOT SUFFERE A CHANGE IN CHEMICAL STRUCTURE AND TOGETHER WITH THE MATRIX MATERIAL EXHIBITS A UNIFORM PLASTICITY, AND PRESSING THE CONTENTS OF THE CAVITY IN THE DIE WHILST AT THAT EVEN TEMPERATURE CONDITION SO THAT THE DIE CONTENTS ARE COMPACTED THROUGHOUT THE DIE CAVITY.

'I J'nitecl States Patent 01 fice 3,845,178 Patented Oct. 29, 1974 3,845,178 MANUFACTURE OF FUEL FOR NUCLEAR REAC- TORS UTILIZING A POLYMERISABLE RESIN Milan Franc Hrovat, Rodenbach, Germany, and John Richard Cox Cough and Michael Stuart Thomas Price, Dorset, England, assignors to United Kingdom Atomic Energy Authority, London, England No Drawing. Filed Nov. 1, 1971, Ser. No. 194,488 Claims priority, application Great Britain, Nov. 13, 1970, 54,278/ 70 Int. Cl. C21c 21/02 US. Cl. 264-5 9 Claims ABSTRACT OF THE DISCLOSURE A method of making pressed compacts of nuclear fuel in the form of fission product retaining coated particles and a matrix material incorporating a polymerisable resin is described in which the pressing is carried out with the compact in a substantially isothermal condition during which the resin melts but does not change its structure chemically. Preferably for this purpose a special phenol formaldehyde is employed.

BACKGROUND OF THE INVENTION This invention relates to the manufacture of fuel for nuclear reactors. For certain kinds of nuclear reactors it has been proposed to use fission product retaining fuel in the form of solid compacts, each compact comprising a certain number of nuclear fuel kernels bearing fission product-retentive coatings and held in a firm matrix of a suitable material. The percentage of the volume of compact occupied by coated fuel particles is termed the fuel volume loading and for high temperature thermal reactors a loading of, say, 35% is typical. The matrix material should be such as to give the compact a good dimensional stability during its lifetime in the reactor and should preserve its integrity during this period and during its charge into, and discharge from the reactor. In the case of a graphite matrix, for example, this means that the matrix must be nearly isotropic, of large crystallite size and of good chemical stability at high temperature in the presence of oxidising impurities.

It has previously been proposed to consolidate a mixture of fission product-retaining fuel particles in a matrix of graphitic powder, the grains of which had been precoated with a phenolic resin-hexamine mixture. In this prior process, the graphite grains and fuel particles were placed in a die and heated until the resin softened. Applied pressure was then effective to consolidate the die contents during which time it was expected that resinated graphite would be sufficiently fluid to aid in distributing the applied pressure uniformly throughout the mass. However the results of this process and variants thereof, particularly those which deal with compacts with high volume loadings were entirely satisfactory. It was found, for instance, there was a risk of the fission product retaining coatings cracking under the pressing pressure. The cause of cracking was not attributed to any one parameter but the high pressing pressure was no doubt a factor involved. A contributory factor in increasing the fracture risk would also seem to be the possibility of non-uniform viscosity (or plasticity) of the matrix forming material at the time of pressing, for such non-uniformity restricts the even distribution of pressure through the matrix material and can set up points of high stress where adjacent particles are in close proximity.

SUMMARY OF THE INVENTION According to the present invention a mixture including fission product retaining fuel particles, a polymerisable resin and granular matrix material are pressed in a die cavity under substantially isothermal conditions at a temperature at which the resin melts but does not suffer a change in chemical structure and, together with the matrix material, exhibits a plasticity which allows compaction throughout the die cavity.

Temperature conditions which are sufficiently uniform for performing the invention may be achieved by heating the die, die plungers and die contents in an oven at a slow rate, e.g. at rates of up to about 4 C. per minute. Faster rates may be permitted, however, depending upon the size of the die, and the resin used. To compensate for the adverse effect which a slow heating rate may have upon the economics of quantity production of compacts, a multiple die body is envisaged in which a large number of die cavities are heated simultaneously.

It is advantageous to employ a polymerisable resin which is chemically unchanged during the heating and pressing stage. A phenol-formaldehyde resin, having a low ortho content, is suitable; the formaldehyde/phenol molecular ratio therein is preferably less than one but nevertheless close to unity. In some circumstances, a hardener such as hexamine, could be added to the resin but care must be taken in this regard to avoid premature polymerisation of the resin binder, for example during the isothermal conditions of pressing to compact the die contents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order that the invention may be better understood one example of a process embodying the invention will now be described.

Nuclear fuel kernels of uranium oxide of an average diameter 800 m and having a porosity of 20% were coated with multiple layers of fission product retentive material to a total coating thickness of 216 mg. The density of the coated particles was 4.2 g./cc. and their uranium content represented 60% of their total weight. A 1 kg. batch of these coated particles were then overcoated with 700 grams of graphite matrix forming powder. The matrix forming material was in fact a graphitised petroleum coke powder precoated with 12% by weight of solid phenol formaldehyde resin.

The resin was specially selected so as to remain chemically unchanged at temperatures at which it exhibits low viscosity and high plasticity to the extent that the pressing pressure to consolidate the final compact can be exceeding low. The selected resin had a high molecular weight, a low free phenol content, i.e. less than 1%, and a formaldehyde/phenol molecular ratio of less than one but nevertheless close to unity. No hardener was added. A suitable phenol-formaldehyde resin has typically the following properties:

Ash content p.p.m 157 Free phenol percent 0.12 Acid number 7.5 pH value 6.0 Melting point vC 97.0 Molecular weight 690.0 Viscosity of a 50% solution in CH CHO'I-I Proportion of the following linkages in the resin:

Para-para percent 4.9 Ortho-para do 2.4 Ortho-ortho do 06 By the use of such a resin particle coating, fracture may be avoided.

The overcoating process itself was performed in accordance with the method described in U. K. Pat. No. 1,081,- 447, a batch of coated particles being rolled in a rotating drum into which the matrix forming material and methylated spirits were sprayed such that the coated particles acquired an overcoating of matrix forming material. The particle diameter was thus increased from an average of 1230 to 1750 Ill 1.. These were then dried over-night at 50 under 200 mm. pressure of flowing nitrogen.

A three-part die block was prepared having 30 parallel cylindrical cavities in one part and upper and lower punches. Aliquots of 14.5 gram overcoated particles were weighed and with the upper punches removed but with the lower punches in position, the die cavities were each charged with this quantity of overcoated particles having the correct filler/fuel proportions for the desired volume loading of 39%. The die was then closed with the punches entering their respective die cavities and the whole block was placed in an oven. The block and contents were then heated slowly and uniformly to 150 C. during 30 minutes. It was then removed from the oven and placed between the thermally insulated platens of a hydraulic press. The press was operated so that all 30 upper and lower punches pressed the contents of their respective die cavities and applied a pressure of about 75 kg./cm. to the die contents under isothermal conditions. The shape of the die and the positions of the end stops were such that cylindrical compacts 11.92 mm. diameter and 50.4 mm. length were formed.

The die was then locked and the block was further heated to 250 C. at which temperature the resin was cured.

The die was removed from the oven and cooled in air. When the temperature had dropped to 100 C. the compacts were ejected from the die.

The volume loading of the coated particles in the compacts was found to be 39% corresponding to a heavy metal content of 0.98 g./cc. of compact volume. The matrix density in the compacts was 1.77 g./cc. giving good thermal conductivity between the coated particles and the exterior of the compact.

The compacts were then heat treated in 2 stages:

(a) to 850 C. in flowing nitrogen to carbonise the resin;

and

(b) to 1800 C. in a vacuum to outgas the compact structure.

Measurements of the ratio of the free uranium to total uranium present in a number of the compacts, fabricated in accordance with the above example, were made by leaching out any free uranium with acid. The ratios obtained were about l which indicates that no particles were broken, the free uranium being that present due to normal contamination on the surface of the coated particles at the start of the consolidation process.

A production line operating on this principle has a number of adjacent stations through which the die blocks are passed in succession. At a first station, a die block has its cavities filled with coated particles and matrix material (preferably overcoated on to the particles). Adjacent to the first station is the sliding entry door to a main oven so that a filled die block can be moved directly into the oven maintained at 320 C. When the temperature of the die block measures 150 C. platens within the oven force the plungers into the dies at the requisite pressure and the plungers are locked in position. In the next die position, the temperature of the block rises to 250 C., to cure the resin.

As mentioned above, for quantity production of compacts, compensation for the slow heating rate (to achieve pressing under substantially isothermal conditions) is obtained by the use of multiple die blocks and the heat is applied by convection to the whole die block including the die plungers in a gradual manner.

We claim:

1. A method of making pressed compacts of nuclear fuel residing in preparing a mixture including fission product retaining fuel particles, a solid polymerisable resin and a granular matrix material, loading a die cavity with the mixture, heating the die and contents of the cavity so that a uniform temperature exists throughout the contents, being a temperature at which the resin melts but does not suffer a. change in chemical structure and together with the matrix material exhibits a uniform plasticity, and pressing the contents of the cavity in the die whilst at that even temperature condition so that the die contents are compacted throughout the die cavity.

2. A method of making pressed compacts of nuclear fuel as claimed in claim 1 in which a die having multiple cavities is employed and the heating of the die and the contents of the cavities is carried out at a slow rate.

3. A method of making pressed compacts of nuclear fuel as claimed in claim 1 in which the polymerisable resin is a phenol formaldehyde resin having a low ortho content.

4. A method as claimed in claim 3 in which a hardener such as hexamine is incorporated in the mixture in a proportion which does not provoke premature polymerisa tion of the resin.

5. A method as claimed in claim 4 in which the phenol formaldehyde resin has a formaldehyde/phenol molecular ratio of less than unity.

6. A method as claimed in claim 3 in which the resin has a high molecular weight and the free phenol content is less than 1%.

7. A method as claimed in claim 1 including the steps of charging the mixture into a plurality of die cavities in a die block, closing each of the cavities and heating the block slowly and uniformly in an oven to about C.

8. A method as claimed in claim 7 which resides in placing the heated die block between the thermally insulated platens of a press and operating the latter to press the contents of all the die cavities simultaneously.

9. A method as claimed in claim 8 in which, after pressing, the die cavities are maintained closed whilst the block is further heated to cure the resin.

References Cited UNITED STATES PATENTS 3,309,433 3/1967 Roberts 2640.5 3,202,619 8/1965 LeBaron 26429 3,261,892 7/1966 Sommer et al. 26429 3,716,609 2/1973 Trocciola et al. 264122 3,708,559 1/1973 Voice et al. 2640.5

STEPHEN J. LECHERT, JR., Primary Examiner B. HUNT, Assistant Examiner US. Cl. X.R. 26429, 122 

1. A METHOD OF MAKING PRESSED COMPACTS OF NUCLEAR FUEL RESIDING IN PREPARING A MIXTURE INCLUDNG GISSION PRODUCT RETANING FUEL PARTICLES, A SOLID POLYMERISABLE RESIN AND A GRANULAR MATRIX MATERIAL, LOADING A DIE CAVITY WITH THE MIXTURE, HEATING THE DIE AND CONTENTS OF THE CAVITY SO THAT A UNIFORM TEMPERATURE EXISTS THROUGHOUT THE CONTENTS, BEING A TEMPERATURE AT WHICH THE RESIN MELTS BUT DOES NOT SUFFERE A CHANGE IN CHEMICAL STRUCTURE AND TOGETHER WITH THE MATRIX MATERIAL EXHIBITS A UNIFORM PLASTICITY, AND PRESSING THE CONTENTS OF THE CAVITY IN THE DIE WHILST AT THAT EVEN TEMPERATURE CONDITION SO THAT THE DIE CONTENTS ARE COMPACTED THROUGHOUT THE DIE CAVITY. 